[PATCH v2 3/4] exports(5): Describe the xprtsec= export option

[Chuck Lever <cel@kernel.org>]

From: Chuck Lever <chuck.lever@oracle.com>

Cc: Rick Macklem <rick.macklem@gmail.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 utils/exportfs/exports.man |   51 +++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 50 insertions(+), 1 deletion(-)

diff --git a/utils/exportfs/exports.man b/utils/exportfs/exports.man
index 54b3f8776ea6..83dd6807c570 100644
--- a/utils/exportfs/exports.man
+++ b/utils/exportfs/exports.man
@@ -125,7 +125,55 @@ In that case you may include multiple sec= options, and following options
 will be enforced only for access using flavors listed in the immediately
 preceding sec= option.  The only options that are permitted to vary in
 this way are ro, rw, no_root_squash, root_squash, and all_squash.
+.SS Transport layer security
+The Linux NFS server allows the use of RPC-with-TLS (RFC 9289) to
+protect RPC traffic between itself and its clients.
+Alternately, administrators can secure NFS traffic using a VPN,
+or an ssh tunnel or similar mechanism, in a way that is transparent
+to the server.
 .PP
+To enable the use of RPC-with-TLS, the server's administrator must
+install and configure
+.BR tlshd
+to handle transport layer security handshake requests from the local
+kernel.
+Clients can then choose to use RPC-with-TLS or they may continue
+operating without it.
+.PP
+Administrators may require the use of RPC-with-TLS to protect access
+to individual exports.
+This is particularly useful when using non-cryptographic security
+flavors such as
+.IR sec=sys .
+The
+.I xprtsec=
+option, followed by an unordered colon-delimited list of security policies,
+can restrict access to the export to only clients that have negotiated
+transport-layer security.
+Currently supported transport layer security policies include:
+.TP
+.IR none
+The server permits clients to access the export
+without the use of transport layer security.
+.TP
+.IR tls
+The server permits clients that have negotiated an RPC-with-TLS session
+without peer authentication (confidentiality only) to access the export.
+Clients are not required to offer an x.509 certificate
+when establishing a transport layer security session.
+.TP
+.IR mtls
+The server permits clients that have negotiated an RPC-with-TLS session
+with peer authentication to access the export.
+The server requires clients to offer an x.509 certificate
+when establishing a transport layer security session.
+.PP
+If RPC-with-TLS is configured and enabled and the
+.I xprtsec=
+option is not specified, the default setting for an export is
+.IR xprtsec=none:tls:mtls .
+With this setting, the server permits clients to use any transport
+layer security mechanism or none at all to access the export.
 .SS General Options
 .BR exportfs
 understands the following export options:
@@ -581,7 +629,8 @@ a character class wildcard match.
 .BR netgroup (5),
 .BR mountd (8),
 .BR nfsd (8),
-.BR showmount (8).
+.BR showmount (8),
+.BR tlshd (8).
 .\".SH DIAGNOSTICS
 .\"An error parsing the file is reported using syslogd(8) as level NOTICE from
 .\"a DAEMON whenever